U.S. patent application number 12/190423 was filed with the patent office on 2010-02-18 for apparatus for stabilizing vertebral bodies.
This patent application is currently assigned to Blackstone Medical Inc.. Invention is credited to Francesco Alfredo Larosa, Mark Evald Semler.
Application Number | 20100042152 12/190423 |
Document ID | / |
Family ID | 41669276 |
Filed Date | 2010-02-18 |
United States Patent
Application |
20100042152 |
Kind Code |
A1 |
Semler; Mark Evald ; et
al. |
February 18, 2010 |
Apparatus for Stabilizing Vertebral Bodies
Abstract
A dynamic stabilization apparatus comprises elongated members
mounted within the proximal end of anchoring devices that are
placed in adjacent vertebral bodies. A flexible element having
elastic properties within the applicable range of loading, for
example loads that the spine experiences, is disposed between the
proximal ends of the elongated members. At least one additional
flexible element is mounted about the proximal ends of the
elongated members adjacent the central flexible element. A housing
encapsulates the proximal ends of the members such that the
flexible element and the additional flexible elements are contained
therein. As compressive, tensile, angular, shear and rotational
forces are applied to the elongated members the central flexible
element and the additional flexible elements interact with the
elongated members and the housing to allow for motion of the
elongated members.
Inventors: |
Semler; Mark Evald; (Morris
Plains, NJ) ; Larosa; Francesco Alfredo; (Neptune,
NJ) |
Correspondence
Address: |
WOODCOCK WASHBURN LLP
CIRA CENTRE, 12TH FLOOR, 2929 ARCH STREET
PHILADELPHIA
PA
19104-2891
US
|
Assignee: |
Blackstone Medical Inc.
Springfield
MA
|
Family ID: |
41669276 |
Appl. No.: |
12/190423 |
Filed: |
August 12, 2008 |
Current U.S.
Class: |
606/250 ;
606/246; 606/254 |
Current CPC
Class: |
A61B 17/7025 20130101;
A61B 17/7023 20130101; A61B 17/7031 20130101; A61B 17/7004
20130101 |
Class at
Publication: |
606/250 ;
606/246; 606/254 |
International
Class: |
A61B 17/70 20060101
A61B017/70 |
Claims
1. An apparatus comprising: at least two elongated members having a
distal and proximal end; a first flexible element disposed between
the proximal ends of the at least two elongated members; at least
one additional flexible element having a collar mounted about the
proximal end of at least one of said elongated members adjacent the
first flexible element; and a housing having a first and a second
end encapsulating the proximal ends of said members such that the
first flexible members and the at least one additional flexible
elements are contained therein wherein the collar protrudes through
an opening in the end of the housing.
2. The apparatus of claim 1 wherein the proximal end of the at
least two elongated members is larger than the distal end.
3. The apparatus of claim 1 wherein the first flexible element is
constructed from a flexible polymer.
4. The apparatus of claim 3 wherein the flexible polymer comprises
polycarbonate polyurethane.
5. The apparatus of claim 1 wherein the at least one additional
flexible element is constructed from a flexible polymer.
6. The apparatus of claim 5 wherein the flexible polymer comprises
polycarbonate polyurethane.
7. The apparatus of claim 1 wherein the housing has at least one
opening therein such that the first flexible element, the at least
one additional flexible element and proximal ends of the elongated
members can be placed therein.
8. The apparatus of claim 1 wherein the housing comprises a first
and a second interlocking member, wherein said members have an
opening therein such that the distal end of said elongated members
may be placed there through.
9. The apparatus of claim 2 wherein the proximal end comprises a
flange.
10. The apparatus of claim 9 wherein the flange comprises an inward
surface facing the first flexible element and an outward surface
facing the distal end of said elongated member.
11. The apparatus of claim 10 wherein the inward surface is
generally concave and contacts a surface of the first flexible
element.
12. The apparatus of claim 10 wherein the outward surface is
generally convex and contacts a surface of the at least one
additional flexible element.
13. The apparatus of claim 10 wherein the first flexible member has
a first and a second outward facing surface and the inward surface
of each of the flanges contact the outer facing surfaces of the
first flexible element to form an anti-torsional coupling.
14. The apparatus of claim 13 wherein the anti-torsional coupling
further comprises at least one protrusion on each of the outer
facing surfaces of the first flexible element and at least one
corresponding recess on the inward surface of each of the flanges
that receives said at least one protrusion.
15. The apparatus of claim 13 wherein the anti-torsional coupling
further comprises at least one recess on each of the outer facing
surfaces of the first flexible element and at least one
corresponding protrusion on the inward surface of each of the
flanges that receives said at least one recess.
16. The apparatus of claim 14 wherein the at least one protrusion
on a first of the outward facing surfaces of the first flexible
element are oriented so as to be offset from the at least one
protrusion on a second of the outward facing surfaces of the first
flexible element at a pre-determined angle.
17. The apparatus of claim 14 wherein the at least one protrusion
comprises a rib that transverses the outward facing surface.
18. The apparatus of claim 14 wherein the at least one protrusion
comprises at least two transverse ribs that intersect.
19. The apparatus of claim 14 wherein the at least one protrusion
comprises at least one polygon.
20. The apparatus of claim 10 wherein the at least one additional
flexible element is mounted about the proximal end of each
elongated member so as to contact the outer surface of the
flange.
21. The apparatus of claim 20 wherein an inward facing surface of
the at least one additional flexible element is convex and the
outer surface of the flanges are generally concave.
22. The apparatus of claim 20 wherein an inward facing surface of
the at least one additional flexible element is concave and the
outer surface of the flanges are generally convex.
23. The apparatus of claim 20 wherein as the elongate members are
moved apart the at least one additional flexible element contacts
the housing and protrudes through a central opening therein so as
to gradually restrict the axial movement of said members.
24. The apparatus of claim 1 wherein the at least two elongated
members are constructed from a rigid material.
25. The apparatus of claim 1 wherein the at least two elongated
members are constructed from a semi-rigid material.
26. A stabilization apparatus for implantation into a spine
comprising: a first and a second elongated member each having a
distal and proximal end; an enlarged region located at the proximal
end of each elongated member, each enlarged region having an inner
and an outer face; a first flexible element disposed between the
enlarged regions of each elongated member; a first and second
additional flexible element mounted about the proximal end of each
elongated member in proximity with the enlarged regions; and a
housing encapsulating the first flexible element, the enlarged
regions, and the additional flexible elements wherein the first
flexible element and additional flexible elements interact with the
enlarged regions and the housing to permit movement of the
elongated members in a direction complimentary to the movement of
the spine.
27. The apparatus of claim 26 wherein the first flexible element
further comprises a protrusion that corresponds to a recess in the
inner face of the enlarged region.
28. The apparatus of claim 27 wherein the first flexible element is
mounted between the inner faces such that the protrusion sits
within the recess.
29. The apparatus of claim 26 wherein the elongated members
protrude through an opening in a first and second end of the
housing such that a space exists between the housing and the
elongated members.
30. The apparatus of claim 29 wherein the first and second
additional flexible elements further comprise a collar.
31. The apparatus of claim 30 wherein the first and second
additional flexible elements are mounted about the elongated
members such that collar sits within the space between the
elongated members and the housing.
32. A vertebral stabilization apparatus, comprising: a housing; a
first and second member for attachment to a first and second
vertebral body in a generally longitudinally aligned orientation; a
means for providing resistance while permitting relative
displacement between the first and second members in a first,
second and a third directions, the first direction being in the
longitudinal direction in which the first and second members are
generally aligned, the second direction being in the lateral
direction approximately perpendicular to the longitudinal
direction, and the third direction being the rotational direction
around the longitudinal direction.
33. An vertebral stabilization apparatus comprising: at least first
and second elongated members each having a distal end and a
proximal end; a first flexible element disposed between the
proximal ends of the at least two elongated members, wherein the
first flexible element undergoes compression as a result of at
least one of the elongated members moving toward the other
elongated member; at least one additional flexible element having a
collar mounted about the proximal end of at least said first
elongated member proximate the first flexible element, wherein the
additional flexible element undergoes compression as a result of
the first elongated member moving away from the second elongated
member; and a housing having a first and a second end and
encapsulating the proximal ends of said elongated members such that
the first flexible members and the at least one additional flexible
elements are contained therein, the first elongated member
extending through an opening in one of the housing ends, at least a
portion of the collar disposed in the opening between the first
elongated member and the housing, wherein the collar undergoes
compression when the first elongated member moves laterally with
respect to the second elongated member.
Description
TECHNICAL FIELD
[0001] The present invention relates to stabilization of the
vertebrae of the spinal column and, more particularly, to an
apparatus whereby securing members are implanted and fixed into a
portion of a patient's spinal column and a longitudinal member
including flexible, semi-rigid rod-like structures of various
cross-sections (hereinafter referred to as "rods") are connected
and fixed to the upper ends of the securing members to provide
stabilization of the spinal column.
BACKGROUND
[0002] Degenerative spinal column diseases, for example, disc
degenerative diseases (DDD), spinal stenosis, and spondylolisthesis
can be corrected by surgical procedures. Typically, spinal
decompression is the first surgical procedure that is performed and
results in the reduction of pressure in the spinal canal and on
nerve roots located therein. Spinal decompression seeks to remove
tissue that is applying pressure to the nerve bundle and thus
relieve pain. This can result, however, in weakening the spinal
column.
[0003] Certain surgical procedures, for example posterolateral
fusion whereby adjacent vertebral bodies are fused together is
often necessary to restore spinal stability following the
decompression procedure. Fusion of adjacent vertebral bodies
requires that the bone grow together and employs a bone graft or
other biological growth agent. In order to maintain the grafting
material in place and preserve stability during bone growth, a
spinal fixation device is typically used to support the spinal
column until a desired level of fusion is achieved. Depending on a
patient's particular circumstances and condition, a spinal fixation
surgery can sometimes be performed immediately following
decompression, without performing the fusion procedure. The
fixation surgery is performed in most cases because it provides
immediate postoperative stability and, if fusion surgery has also
been performed, it provides support of the spine until sufficient
fusion and stability has been achieved.
[0004] Conventional methods of spinal fixation utilize a rigid
spinal fixation device to support and prevent movement of an
injured spinal part. These conventional spinal fixation devices
include: fixing screws configured to be inserted into the spinal
pedicle or sacrum to a predetermined depth and angle, rods or
plates configured to be positioned adjacent to the injured spinal
part, and coupling elements for connecting and coupling the rods or
plates to the fixing screws such that the injured portion of the
spin is supported and held in a relatively fixed position by the
rods or plates. The connection units prevent further pain and
injury to the patient by substantially restraining the movement of
the spinal column.
[0005] Because the connection units prevent normal movement of the
spinal column, after prolonged use, the spinal fixation device can
cause ill effects, such as adjacent level syndrome (transitional
syndrome) or fusion disease that result in further complications
and abnormalities associated with the spinal column. The high
rigidity of the rods or plates used in conventional fixation
devices causes these disorders due to the patient's joints being
fixated by the nature of surgery. The movement of the spinal joints
located above or under the operated area is increased.
Consequently, such spinal fixation devices cause decreased mobility
of the patient and increased stress and instability to the spinal
column joints adjacent to the operated area.
[0006] It has been reported that excessive rigid spinal fixation is
not helpful to the fusion process due to load shielding. As an
alternative, semi-rigid spinal fixation devices have been utilized
to address this problem while assisting the bone fusion process.
For example, U.S. Pat. No. 5,375,823--Navas and U.S. Pat. No.
6,241,730--Alby each disclose a piston configuration mounted
between fixing screws having a flexible material or spring element
enclosed within a sleeve allowing for axial dampening. Although
providing for a greater range of motion than a fixed rod, these
devices fail to accommodate for a full range of physiological
motion, for example axial torsion or twisting, and are not
well-suited for spinal stabilization absent fusion. Thus, in the
end these devices do not fully prevent the problem of rigid
fixation resulting from fusion.
[0007] To solve the above-described problems associated with rigid
fixation, semi-rigid and generally flexible devices have been
developed. U.S. Publication No. 2006/0264940--Hartmann discloses a
flexible spring element connected to a rod and an axially opposed
hollow body. The spring element and hollow body have corresponding
bores that receive a clamping element. The clamping element has a
convex face that abuts the end wall of the internal bore of the
spring element during deformation of the spring element under axial
loading of the device. The shape of the end of the clamping element
controls the spring characteristics of element. While this device
functions to provide a greater range of motion during compression
it relies upon the spring element as a load bearing structure in
tension. This is not an optimal design to handle the long-term
cyclical loading the device will experience when implanted.
[0008] U.S. Pat. No. 5,672,175--Martin discloses a flexible spinal
fixation device which utilizes a flexible rod made of metal alloy
and/or a composite material. Additionally, compression or extension
springs are coiled around the rod for the purpose of providing
de-rotation forces on the vertebrae in a desired direction.
However, this approach is primarily concerned with providing a
spinal fixation device that permits "relative longitudinal
translational sliding movement along [the] vertical axis" of the
spine and has a solid construction with a relatively small diameter
in order to provide a desired level of flexibility. Because they
are typically very thin to provide suitable flexibility, such a rod
is prone to mechanical failure and have been known to break after
implantation in patients. Similarly, U.S. Publication No.
2007/0270814--Lim shows a vertebral stabilizer that has mobility
during compression, extension and rotation. A connecting member
such as flexible rods, cables or braided steel are anchored at
their distal and proximal ends to engaging portions and are
coaxially located within a flexible member. While the connecting
members can bend to accommodate shear when the spine is twisted
this device has been shown to fail due to fatigue once
implanted.
[0009] There is no spinal fixation device that can provide for a
full range of physiological motion when implanted in a patient. In
addition, few devices that attempt to accommodate a range of
physiological motion can withstand long-term loading conditions.
Therefore, there is a need for an improved dynamic spinal fixation
device.
SUMMARY
[0010] Elongated members such as rods, plates and the like are
often mounted to span vertebral bodies in order to provide
stability to localized regions of the spine. These devices are
typically mounted to the vertebral bodies via an anchoring device
such as a member having threads at its distal end, allowing for
attachment to the spine and a proximal end that accepts the
elongated member. For example, at least two threaded members are
placed in adjacent vertebral bodies and the elongated members are
mounted to the proximal end of threaded members so as to span the
vertebral bodies. Rigid elongated bodies are typically employed in
order to prevent motion between the vertebral bodies.
[0011] According to the invention, a dynamic stabilization
apparatus is provided. The apparatus comprises elongated members
such as rods mounted within a housing. The elongated members are
mounted within the proximal end of anchoring devices that are
placed in adjacent vertebral bodies. A central flexible element
having elastic properties within the applicable range of loading,
for example loads that the spine experiences, is disposed between
the proximal ends of the elongated members. At least one additional
flexible element is mounted about the proximal ends of the
elongated members adjacent the central flexible element. The
housing encapsulates the proximal ends of the members such that the
central flexible element and the additional flexible elements are
contained therein. As compressive, tensile, angular, shear and
rotational forces are applied to the elongated members the central
flexible element and the additional flexible elements interact with
the elongated members and the housing to allow for motion of the
elongated members. The degree of permissible motion may be varied,
for example, by varying the material from which the flexible
members are constructed.
[0012] The housing may be generally cylindrical and has openings at
each end for receiving the elongated members there through. In one
embodiment of the invention, the housing is constructed from a
generally rigid material that will not deform under the
physiological loading encountered within the spine. The housing may
be formed from a first and a second casing wherein each of the
casings have an opening therein. The casings include an engagement
feature such that after the elongated members are inserted through
the openings the casings are engaged together to assemble the
apparatus.
[0013] In one embodiment of the invention the proximal ends of the
elongated elements are larger than the central and distal portions
of the elongated member. For example, one or both of the proximal
ends are flanges. The flange includes an inward surface facing the
central flexible element and an outward surface facing the distal
end of said elongated member. The inward surface may be generally
concave and contacts an outer facing surface of the central
flexible element that is convex. Alternatively, the inner surface
of the flange may be convex while the contacting or outer surface
of the central flexible element is concave. A variety of shapes for
the two surfaces may be employed including having both surfaces be
flat.
[0014] The central flexible member may be constructed from a
polymer and has a first and a second outward facing surface. The
central flexible member resists rotational and compressive forces.
The inward surface of each of the flanges contacts the outer facing
surfaces of the central flexible element. Protrusions located on
either the outward surface of the central element or the inward
surface of the flanges engages with corresponding recesses to form
an anti-torsional coupling. As the elongated members are rotated
about their axis in opposite directions the engagement of the
protrusions within the recesses causes the central flexible element
to elastically deform, resisting the motion. In addition, as the
elongated members experience a compressive force the flanges engage
the central flexible element. The central flexible element is
compressed resisting while allowing motion of the elongate members.
Eventually the central flexible element deforms such that it
contacts the housing further increasing the resistance to the
motion of the elongated members.
[0015] The outward surface of the proximal end of the elongate
members or the flange contacts a surface of the additional flexible
element. A variety of shapes can be employed for the outward
surface of the flange and the corresponding contacted surface of
the additional flexible element. The shaping of these surfaces may
be varied in order to create a desired dynamic response. As with
the central element, the additional flexible elements may be
constructed from a polymer. The additional flexible elements serve
as an axial and radial buffer between the housing and the elongated
members. For example, as the elongated members are subjected to an
axial or radial force, the flange pushes on and deforms the
additional flexible member which resists the motion of the
elongated members. Varying the elastic properties of the central
and flexible members allows the load-displacement response of the
apparatus to be customized.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The features and advantages of the invention will be
apparent to those of ordinary skill in the art from the following
detailed description of which:
[0017] FIG. 1 is an isometric view of to an embodiment of the
present invention;
[0018] FIG. 2 is an exploded view of the components of an
embodiment of the present invention.
[0019] FIG. 3 is a side view of an embodiment of the present
invention.
[0020] FIG. 4 is a view of an embodiment of the present invention
taken along line 4-4 of FIG. 3.
[0021] FIG. 5 is an isometric view of an embodiment of the central
element of the present invention.
[0022] FIG. 6 is an alternative embodiment of the central element
of the present invention.
[0023] FIG. 7 is an alternative embodiment of the central element
of the present invention.
[0024] FIG. 8 is an alternative embodiment of the central element
of the present invention.
[0025] FIG. 9 is an alternative embodiment of the central element
of the present invention.
[0026] FIG. 10A is a cross section view showing an embodiment of
the present invention in an unloaded state.
[0027] FIG. 10B is a cross section view showing an embodiment of
the present invention under tension.
[0028] FIG. 11A is a cross section view showing an embodiment of
the present invention in compression.
[0029] FIG. 11B is a cross section view showing an embodiment of
the present invention in a further compressed state.
[0030] FIG. 12 is a posterior view showing an embodiment of the
present invention placed on a section of the spine.
[0031] FIG. 13 is a side view showing an embodiment of the present
invention placed on a section of the spine whereby the section of
the spine is in flexion.
[0032] FIG. 14 is a side view showing an embodiment of the present
invention placed on a section of the spine whereby the section of
the spine is in extension.
[0033] FIG. 15 is a posterior view showing an embodiment of the
present invention placed on a section of the spine whereby the
section of the spine experiences lateral bending.
[0034] FIG. 16 is a posterior view showing an embodiment of the
present invention placed on a section of the spine whereby the
section of the spine experiences axial rotation.
[0035] FIG. 17 is a cross section view showing an embodiment of the
present invention actuated under shear.
[0036] FIG. 18 is a cross section view showing an embodiment of the
present invention angulated.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0037] An implantable dynamic apparatus for stabilizing a desired
region of the spine will be described with reference to FIGS. 1-18.
As shown in FIGS. 1-4 the apparatus 10 of the present invention
generally comprises elongated members 40a, b mounted within a
housing 20. A central element 50 having elastic properties is
disposed between the proximal ends of the two elongated members
40a, b. At least one additional flexible or compressible element
30a, b is mounted about the proximal end of the elongated members
40a, b adjacent the central element 50. The housing 20 encapsulates
the proximal ends of the elongated members 40a, b such that the
central element 50 and the at least one additional flexible or
compressible element(s) 30a, b are contained therein.
[0038] The elongated members 40a, b may be constructed from
materials having sufficient strength and rigidity to resist
fracture and plastic deformation under the loads experienced by the
spine. Materials such as titanium, titanium alloy, stainless steel
or a polymer such as PEEK or carbon fiber may be employed. The
elongated members 40a, b may have a variety of shapes such as
cylindrical or polygonal and need not both have the same shape. The
construction of the components of the apparatus 10 may be varied to
meet the particular conditions of the patient in which the
apparatus 10 will be utilized. For example, the material used to
construct the central element 50 may be varied or the size and
shape or the elongated members 40a, b can be varied such that each
member has a different shape or is constructed from a different
material.
[0039] As shown in FIG. 12, one or more of apparatus 10 can be
mounted between adjacent vertebral bodies 8a, b in order to provide
stability to localized regions of the spine 2. Typically,
stabilization devices such as apparatus 10 are mounted to the
vertebral bodies 8a, b via an anchoring device 3. The device may
comprise a member having threads at its distal end, not shown in
the drawings that allow for attachment to the boney tissue of the
spine and a proximal end 4 that accepts the distal ends of
elongated members 40a, b. Alternatively, the distal end may
comprise a clamp or other gripping surface. A retaining member 6
locks the distal ends of the elongated members 40a, b to the
anchoring devices 3. As will be described in greater detail below,
when the spine experiences the normal range of physiological motion
the central element 50 and the additional flexible or compressible
elements 30a, b interact with the elongated members 40a, b and the
housing 20 to stabilize the spine while allowing for controlled
movement of the adjacent vertebral bodies 8a, b.
[0040] As shown in FIGS. 2 and 4, the central flexible element 50
is situated between the proximal ends of the elongated members 40a,
b. The shape of the central element 50 is designed to maximize
contact with the proximal ends of the elongated members 40a, b as
described in greater detail below, while leaving a space 70 between
element 50 and the housing 20 to allow for distortion of the shape
of the central flexible element 50 when elongated members 40a, b
are moved inward.
[0041] The central flexible element 50 can be homogeneous or made
as a composite to tailor its performance to the particular loading
apparatus 10 experiences when implanted. In one embodiment of the
present invention, the central flexible element 50 is constructed
from an incompressible elastomer such that as elongated members
40a, b are moved inwards, element 50 experiences transverse strain
in response to axial strain. A flexible material having a durometer
range of 30-65 on the Shore D scale or 20-95 on the Shore A scale
and an elongation at break in the range of 200-600% per ASTM D-638
may be utilized to construct the central flexible element 50. The
material utilized preferably is biocompatible and exhibits a
consistent dynamic response and resists wear over the millions of
loading cycles experience by the apparatus 10 when implanted in the
spine. One such material is Polycarbonated Polyurethane or PCU
known commercially as Chronoflex. One grade of Chronoflex that has
been shown to function with the present invention is Chronoflex C
55D.
[0042] As shown in FIG. 10A the central element is unexpanded when
the apparatus 10 is in a neutral, unloaded position. As shown in
FIGS. 11A and B, when members 40a, b are moved in the direction of
arrows 62, such as would be experience when the spine is extended,
element 50 eventually expands into space 70 contacting the inner
wall of housing 20. With further movement of the members 40a, b,
the element 50 further expands into interstitial space 28. The
expansion of the element 50 into spaces 70 and under certain
conditions space 28 causes an exponential increase in resistance to
compressive loading. This allows for the restricted movement of the
adjacent vertebral bodies that apparatus 10 spans while also
providing stability thereto.
[0043] As shown in FIG. 2, the proximal ends of the elongated arms
or members 40a, b are larger than the central and distal portions
of the elongated members 40a, b. For example, one or both of the
proximal ends comprise flanges 42a, b. The flanges 42a, b include
outer surfaces 46a, b facing the distal end of the elongated
members 40a, b and inner surfaces 48a, b that face the central
element 50. As shown in FIG. 4, the inner surfaces 48a, b of the
flanges 42a, b may be convex while the opposing surfaces 54a, b of
the central element 50 are concave. Alternatively, the inward
surfaces 48a, b may be generally concave while the opposing
surfaces 54a, b are convex. In addition, a variety of shapes for
the two surfaces may be employed including having both surfaces be
flat or having non complimentary geometries. The shape of the
surfaces 48a, b and 54a, b will impact the angular and shear
displacement between the elongated members 40a, b. The convex and
concave shape of surfaces 48a, b and 54a, b respectively act as a
rotating joint allowing for articulation as apparatus 10
experiences angular and shear loading.
[0044] The central flexible element 50 may include one or more
features on surfaces 54a, b that allows it to interface with the
flanges 42a, b. As shown in FIGS. 2 and 5, the central element 50
includes two ribs 52a, b on the outer surfaces 54a, b. Each flange
42a, b includes a slot 44a, b that corresponds to the geometry of
the ribs 52a, b such that the ribs are received and under certain
conditions engaged therein. As the elongated members 40a, b are
twisted under torsional loading, the ribs 52a, b engage the slots
44a, b acting to resist the twisting movement. The ribs 52a, b are
oriented orthogonally to each other to allow for more consistent
performance in angulation and shear. This also allows for
implantation of the device without regard to orientation.
[0045] Over time the frictional and compressive forces resulting
from the contact between flanges 42a, b and central element 50 will
adversely affect the dynamic performance of element 50 due to wear
and degredation. Although the central element 50 is constructed
from a material that resists wear, the geometry of the features on
surfaces 54a, b may be varied in order to increase the durability
of the central element 50. FIGS. 6-9 illustrate examples of
geometries that may be employed. FIGS. 2 and 6 shows a plurality of
ribs 56a, b disposed on the surface 54a, b such that the amount of
surfaces 54a, b that contact the inner surfaces 48a, b are
minimized. FIG. 7 shows the central flexible element 50 with
centrally located polygons 58a, b. In addition, the polygons 58a, b
may have any number of sides, for example, forming a star with a
plurality of points. As shown in FIG. 8 the central flexible
element 50 has a plurality of polygons 58a, b located at the
perimeter of central element 50. In this embodiments polygons 58a,
b are shown with three sides but could be any number of sides and
need not match each other and not be arranged in any pattern.
[0046] FIG. 9 shows the central flexible element 50 with a
plurality of cylindrical protrusions 60a, b positioned in a pattern
about the surface 54a, b of the central flexible element 50. As
with the other embodiments, the cylindrical protrusions may be
arranged in any manner and need not follow a pattern. In all of the
shown embodiments in FIGS. 5-9, the type of engagement features on
one side of the central flexible element 50 need not match the
engagement features on the opposite side. For example one surface
54a a may have a cruciform 56a while the other surface 54b has
multiple polygons 58b. In addition, the surfaces 54a, b can each
have different types of features such as polygons and cylindrical
protrusions thereon.
[0047] As shown in FIGS. 2 and 4, a flexible element or elements
30a, b is mounted about the proximal end of one or both of the
elongated members 40a, b. The flexible elements 30a, b comprise a
collar 34a, b and a central region 36a, b each interacting with
housing 20 depending upon the movement of the elongated members
40a, b. For example, the central region 36a, b of the flexible
elements 30a, b is acted upon when the elongated members 40a, b are
moved in an axial direction. The collar 34a, b may protrude
slightly beyond the margin of the housing 20 and is acted upon when
the elongated members 40a, b are moved in an angular or radial
direction. As with the central flexible element 50 one or both of
the flexible elements 30a, b may be constructed form a Newtonian
material whereby transverse strain in response to axial strain is
described by Poisson's ratio.
[0048] As shown in FIG. 10A an inner surface 32a, b of the central
region 36a, b of the collar 34a, b contacts an outer surface 46a, b
of the flange 42a, b when the elongated members 40a, b are in a
neutral position, for example, when the spine is at rest.
Alternatively, a space may exist between the surfaces 32a, b and
46a, b, not shown in the drawings, to allow for greater
unrestricted axial movement of elongated members 40a, b. FIG. 10B
illustrates the elongated members 40a, b subjected to an axial
force causing the members 40a, b to move distally in the direction
of arrow 61 such as would be experience when the spine is flexed.
The outer surfaces of flanges 46a, b pushes on the flexible members
30a, b at contact surfaces 32a, b. This in turn causes flexible
members 30a, b to come into contact with the housing 20 and deform
into interstitial space 28 and/or push the collar 34a, b through an
opening in the ends 21a, b of the housing 20. The expansion of the
flexible members 30a, b into space 28 and through the ends 21a, b
of housing 20 causes an exponential increase in resistance to axial
loading. This allows for the restricted and stabilized movement of
adjacent vertebral bodies.
[0049] A variety of shapes and sizes can be employed for the outer
surfaces 46a, b of the flanges 42a, b and the corresponding
contacting surface 32a, b of the flexible elements 30a, b. The
shape of these surfaces may be varied in order to create a desired
dynamic response. Providing a concave shape on surfaces 32a, b may
lead to a more rapid deformation of the flexible elements 30a, b
and, consequentially more rapid stiffening to limit the range of
motion for the elongated members 40a, b. Alternatively, a convex
shape may be utilized whereby the flanges 42a, b have a thinner
profile allowing for the flexible elements 30a, b to be larger.
This may provide for a greater range of motion to the elongated
members 40a, b.
[0050] As shown in FIGS. 1-4 the housing 20 may be generally
cylindrical and has openings at each end 21a, b allowing elongated
members 40a, b to pass there through. In one embodiment of the
invention, the housing 20 is constructed from a generally rigid
material that will not deform under the physiological loading
encountered within the spine. The housing 20 may be formed from a
first 20a and a second 20b casing wherein each of the casings 20a,
20b have an opening at ends 21a, b. As shown in FIG. 4, casing 20a
includes a locking feature 22 that corresponds to a locking feature
24 on casing 20b so as to form a snap lock. Casing 20b also
includes a plurality of expansion slots 26 that aid in assembly of
the apparatus as will be described below.
[0051] As shown in FIG. 2, the apparatus 10 is assembled by
aligning the proximal ends of elongated members 40a, b with the
central flexible element 50 such that the protrusions 52a, b
correspond to slots 44a, b. The flexible elements 30a, b, which
have an opening in the center corresponding to the cross sectional
geometry of elongated members 40a, b, are then inserted over the
distal ends of elongated members 40a, b and slid into place about
the proximal end thereof or in proximity to flanges 42a, b.
Thereafter, the distal ends of elongated members 40a, b are placed
through the openings in the ends 21a, b of casings 20a, b. The
openings in the ends 21a, b are larger than the cross sectional
geometry of the elongated members 40a, b creating a space between
the elongated members 40a, b and the housing 20. Casings 20a, b are
placed together and inward pressure applied thereon such that
locking features 22 slides under locking feature 24 which moves in
a radial direction as facilitated by expansion slots 26 locking
casings 20a, b together. The casings can also be assembled by
welding, bolting, threading, screwing, adhesive bonding, magnetic
coupling, clamping, or twist locking. Once casing 20 is assembled,
the proximal ends of members 40a, b, the central element 50, the
flexible elements 30a, b and the flanges 42a.b are contained
therein. The collars 34a, b of flexible elements 30a, b, however,
are located in the spaces between the housing 20 and members 40a, b
and may slightly protrude through the space between the elongated
members 40a, b and the openings at the ends 21a, b of the housing
20.
[0052] FIG. 12 illustrates the apparatus 10 implanted in the spine
between adjacent vertebral bodies 8a, b in order to provide
stability to the joint existing between vertebrae 8a, b. Typically,
stabilization devices such as apparatus 10 are mounted to the
vertebral bodies 8a, b via an anchoring device 3. Once mounted to
the spine 2 the apparatus 10 serves to stabilize adjacent vertebral
bodies while also allowing for motion.
[0053] As shown in FIGS. 10A-B, when the spine 2 is moved in the
direction of arrows 11 the elongated members 40a, b transfer force
to the flexible elements 30a,b as described above allowing for a
limited range of motion. As shown in FIGS. 11A-B and 14 when the
spine is moved in the direction of arrows 12 or placed in extension
the elongated members 40a, b moved toward the central flexible
element 50. The central flexible element 50 expands into spaces 70
and under certain conditions, for example increased extension of
the spine creating the dynamic response discussed above. As shown
in FIG. 15, the spine 2 is experiencing lateral or side bending, as
indicated by the arrows 13. Under the lateral loading shown in FIG.
15 apparatus 10a will transfer the loading forces to flexible
members 30a, b while the apparatus 10b will transfer loading to the
central flexible element 50.
[0054] FIG. 16 shows the spine rotated about its axis in the
direction of arrow 14. Due to the apparatus 10 being mounted away
from the axis of the spine the device 10 experiences shear loading.
As shown in FIG. 17 the elongated members 40a, b are moved in the
direction indicated by arrows 63. As the elongated members 40a, b
are moved, surfaces 48a, b and 54a, b shift relative to each other
such that flange 42a, b is free to interact with additional
flexible elements 30a, b. As the flanges 42a, b contact additional
flexible elements 30a, b, they impinge upon the housing 20.
Thereafter, the additional flexible elements 30a, b resist movement
in the manner described above with reference to the distal movement
of the elongated members 40a, b. This dynamic response maintains
the elongated members 40a, b roughly parallel to each other as the
central flexible element 50 and the housing 20 rotate.
[0055] Apparatus 10 has been described above primarily with
reference to unidirectional loading conditions. As shown in FIG.
18, however, the apparatus 10 can handle loading in multiple
directions simultaneously. For example, as shown in FIG. 13 the
elongated members 40a, b are moved both distally and obliquely from
each other causing the additional flexible elements 30a, b interact
with the flanges 42a, b and the housing 20 in a manner as described
above. The space between the housing 20 and the elongated members
40a, b into which collars 34a, b are placed allows elongated
members 40a, b a range of angular motion that is restricted in the
direction of movement by the additional flexible elements 30a, b
whereby collars 34a, b act as a buffer between the housing 20 and
the elongated members 40a, b.
[0056] Although the present invention has been described above with
respect to particular preferred embodiments, it will be apparent to
those skilled in the art that numerous modifications and variations
can be made to these designs without departing from the spirit or
essential attributes of the present invention. Accordingly,
reference should be made to the appended claims, rather than to the
foregoing specification, as indicating the scope of the invention.
The descriptions provided are for illustrative purposes and are not
intended to limit the invention nor are they intended in any way to
restrict the scope, field of use or constitute any manifest words
of exclusion.
* * * * *